ON 5:23 P.M. THURSDAY, SEPT. 22 AT CHICAGO

Tom SkillingCHICAGO TRIBUNE

Fall is closing in on northern Illinois quickly. Meteorological autumn began on Sept. 1, and now astronomical autumn begins this Thursday. On that date, the sun crosses the equator having made half of its journey from its highest point in the Northern Hemisphere's sky directly over the Tropic of Cancer at summer solstice, toward its lowest point in the Northern Hemisphere when it will stand directly over the Tropic of Capricorn at the winter solstice. At this time of the year, the loss of daylight accelerates to its maximum rate. In Chicago, each day is about 3 minutes shorter than the day before.

The pendulum effect

The rate of daylight change resembles a pendulum, moving very little at the top of its arc (winter/summer solstice) and accelerating in the middle of its swing (autumnal/spring equinox)

Source: U.S. Naval Observatory

WGN-TV/Paul Dailey

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Tom Skilling is chief meteorologist at WGN-TV. His forecasts can be seen Monday through Friday on WGN-TV News at noon and 9 p.m.